Multi-axial bone screw assembly

ABSTRACT

A bottom-loading multi-axial bone anchor apparatus is disclosed. The apparatus includes a receiver member, a crown member, a bone anchor and a retaining member. The receiver member defines an upper opening and a lower opening, which may form part of the same opening, a channel, and a groove. The crown member and bone anchor are loaded into the lower opening of the receiver member, and the retaining member fits around the bone anchor and into the groove in the receiver member. The bone anchor is capable of multi-axial positioning with respect to the receiver member. An elongated member is placed in the channel of the receiver member, contacting the crown member, and a compression member is applied via the upper opening. The compression member presses down on the elongated member, which presses down on the crown member and locks the bone anchor between the crown member and the retaining member.

FIELD OF THE INVENTION

[0001] The present invention relates to devices and implants used inosteosynthesis and other orthopedic surgical procedures. Specifically,the present invention contemplates a bottom loading bone anchor assemblycapable of achieving multiple angular orientations with respect to anelongated member extending along bone tissue.

BACKGROUND OF THE INVENTION

[0002] Several techniques and systems have been developed for correctingand stabilizing damage or malformation of bones, especially the longbones and the spine. In one type of system, an elongated member such asa bendable rod is disposed longitudinally along a length of the bone(s).In spinal applications, the rod is preferably bent to correspond to thenormal curvature of the spine in the particular region beinginstrumented. For example, the rod can be bent to form a normal kyphoticcurvature for the thoracic region of the spine, or a lordotic curvaturefor the lumbar region. In accordance with such a system, the rod isengaged to various vertebrae along a length of the spinal column by wayof a number of fixation elements. A variety of fixation elements can beprovided which are configured to engage specific portions of thevertebra and other bones. For instance, one such fixation element is ahook that is configured to engage the laminae of the vertebra. Anothervery prevalent fixation element is a screw that can be threaded intovarious parts of the vertebrae or other bones.

[0003] In one typical spinal procedure utilizing a bendable rod, the rodis situated on opposite sides of the spine or spinous processes. Aplurality of bone screws are threaded into a portion of severalvertebral bodies, very frequently into the pedicles of these vertebrae.The rods are affixed to these plurality of bone screws to applycorrective and stabilizing forces to the spine.

[0004] One example of a rod-type spinal fixation system is the TSRH®Spinal System sold by Medtronic Sofamor Danek, Inc. The TSRH® Systemincludes elongated rods and a variety of hooks, screws and bolts allconfigured to create a segmental construct throughout the spine. In oneaspect of the TSRH® System, the spinal rod is connected to the variousvertebral fixation elements by way of an eyebolt. In this configuration,the fixation elements are engaged to the spinal rod laterally adjacentto the rod. In another aspect of the TSRH® System, a variable anglescrew is engaged to the spinal rod by way of an eyebolt. The variableangle screw allows pivoting of the bone screw in a single plane parallelto the plane of the spinal rod. Details of this variable angle screw canbe found in U.S. Pat. No. 5,261,909 to Sutterlin et al., owned by theAssignee of the present invention. One goal achieved by the TSRH® Systemis that the surgeon can apply vertebral fixation elements, such as aspinal hook or a bone screw, to the spine in appropriate anatomicpositions. The TSRH® System also allows the surgeon to easily engage abent spinal rod to each of the fixation elements for final tightening.

[0005] Another rod-type fixation system is the Cotrel-Dubosset/CD®Spinal System sold by Medtronic Sofamor Danek, Inc. Like the TSRH®System, the CD® System provides a variety of fixation elements forengagement between an elongated rod and the spine. In one aspect of theCD® System, the fixation elements themselves include a body that definesa slot within which the spinal rod is received. The slot includes athreaded bore into which a threaded plug is engaged to clamp the rodwithin the body of the fixation element. The CD® System includes hooksand bone screws with this “open-back” configuration. Details of thistechnology can be found in U.S. Pat. No. 5,005,562 to Cotrel. Onebenefit of this feature of the CD® System is that the fixation elementis positioned directly beneath the elongated rod. This helps reduce theoverall bulkiness of the implant construct and minimizes the trauma tosurrounding tissue.

[0006] On the other hand, these fixation elements of the CD® System arecapable only of pivoting about the spinal rod to achieve variableangular positions relative to the rod. While this limited range ofrelative angular positioning is acceptable for many spinal pathologies,many other cases require more creative orientation of a bone screw, forinstance, relative to a spinal rod. Certain aspects of this problem areaddressed by the variable angle screw of the TSRH® System, as discussedin the '909 patent. However, there is a need for a bone screw that iscapable of angular orientation in multiple planes relative to the spinalrod. Preferably, the bone screw is capable of various three-dimensionalorientations with respect to the spinal rod. Screws of this type havebeen referred to as poly-axial or multi-axial bone screws.

[0007] Others have approached the solution to this problem with variouspoly-axial screw designs. For example, in U.S. Pat. No. 5,466,237 toByrd et al., a bone screw is described which includes a sphericalprojection on the top of the bone screw. An externally threaded receivermember supports the bone screw and a spinal rod on top of the sphericalprojection. An outer nut is tightened onto the receiver member to pressthe spinal rod against the spherical projection to accommodate variousangular orientations of the bone screw relative to the rod. While thisparticular approach utilizes a minimum of components, the security ofthe fixation of the bone screw to the rod is lacking. In other words,the engagement or fixation between the small spherical projection on thebone screw and the spinal rod is readily disrupted when theinstrumentation is subjected to the high loads of the spine,particularly in the lumbar region.

[0008] In another approach shown in U.S. Pat. No. 4,946,458 to Harms etal., a spherical headed bone screw is supported within separate halvesof a receiver member. The bottom of the halves are held together by aretaining ring. The top of the receiver halves are compressed about thebone screw by nuts threaded onto a threaded spinal rod. In anotherapproach taken by Harms et al., in U.S. Pat. No. 5,207,678, a receivermember is flexibly connected about a partially spherical head of a bonescrew. Conical nuts on opposite sides of the receiver member arethreaded onto a threaded rod passing through the receiver. As theconical nuts are threaded toward each other, the receiver memberflexibly compresses around the head of the bone screw to clamp the bonescrew in its variable angular position. One detriment of the systems inthe two Harms et al. patents is that the spinal rod must be threaded inorder to accept the compression nuts. It is known that threading rodscan tend to weaken the rods in the face of severe spinal loads.Moreover, the design of the bone screws in the '458 and '678 patentsrequire a multiplicity of parts and are fairly complicated to achievecomplete fixation of the bone screw.

[0009] A further approach illustrated in U.S. Pat. No. 5,797,911 toSherman et al., owned by the Assignee of the present invention, is toprovide a U-shaped holder through the top of which a bone fastenertopped with a crown member is loaded. The holder accommodates a rod in achannel above the crown member and a compression member above the rod.The compression member presses on the rod and crown member to lock thefastener against the holder in any of a number of angles in threedimensions with respect to the rod. This approach has proven to be quiteeffective in addressing the above-identified problems. However, it doesnot permit bottom-loading of the fastener. Additionally, the holder issomewhat bulky in order to accommodate the other structural components.

[0010] Yet a further approach is shown in U.S. Pat. No. 5,733,285 toErrico et al., in which a holder is provided with a tapered andcolletted portion at the bottom into which a bone fastener head isinserted. A sleeve is provided that slides down around the collettedportion to crush lock the colletted portion around the head of the bonefastener. This apparatus is believed to be relatively bulky anddifficult to manipulate given the external sliding locking mechanism. Itis further dependent on the fit of the external sleeve and the relativestrength of the collet and its bending and crushing portions for securelocking of the bone fastener head.

[0011] There is therefore a need remaining in the industry for amulti-axial bone anchor that can be readily and securely engaged to anelongated member of any configuration—i.e., smooth, roughened, knurledor even threaded—which achieves improved angulation of the bone anchor,improved strength, and reduced size, including profile and bulk, of thecomponents used to engage the bone anchor to the elongated member in anyof a variety of angular orientations.

SUMMARY OF THE INVENTION

[0012] In one embodiment of the invention, a bone fixation assembly isprovided that includes a receiver member defining an upper openingportion and a lower opening portion, a channel configured to receive anelongated member that communicates with the upper and lower openingportions, and a groove around a portion of the lower opening portion.The assembly further includes a crown member with an upper surface and alower surface that is movably disposed in the lower opening portion, anda bone-engaging anchor having a lower portion configured to engage abone and a head smaller than the lower opening portion, with the headbeing movably disposed in the lower opening portion adjacent to theconcave lower surface of the crown member. The assembly also includes aretaining member defining an aperture smaller than the head of the boneanchor that is at least partially housed in the groove of the receivermember and positioned around the bone anchor and below its head.

[0013] Additional embodiments, examples, advantages, and objects of thepresent invention will be apparent to those of ordinary skill in thisart from the following specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a side elevational view of one embodiment of themulti-axial bone screw anchor assembly of the present invention.

[0015]FIG. 2 is an exploded view of the embodiment of the inventiondepicted in FIG. 1.

[0016]FIG. 3a is a side elevational view of an embodiment of thereceiver member of the embodiment of the invention illustrated in FIG.2.

[0017]FIG. 3b is a front elevational view of the embodiment of thereceiver member illustrated in FIG. 3a.

[0018]FIG. 3c is a sectional view, taken along the lines 3 c-3 c in FIG.3a, and viewed in the direction of the arrows, of the embodiment of thereceiver member illustrated in FIG. 3a.

[0019]FIG. 3d is a sectional view, taken along the lines 3 d-3 d of FIG.3b and viewed in the direction of the arrows, of the embodiment of thereceiver member illustrated in FIG. 3a.

[0020]FIG. 4a is a side elevational view of an embodiment of a boneanchor used in the embodiment of the invention illustrated in FIG. 2.

[0021]FIG. 4b is a sectional view, taken along the lines 4 b-4 b of FIG.4a and viewed in the direction of the arrows, of the embodiment of thebone anchor illustrated in FIG. 4a.

[0022]FIG. 4c is a magnified view of one embodiment of the head of theembodiment of the bone anchor illustrated in FIG. 4a.

[0023]FIG. 5a is a top view of one embodiment of a crown member used inthe embodiment of the present invention illustrated in FIG. 2.

[0024]FIG. 5b is a sectional view, taken along the lines 5 b-5 b in FIG.5a and viewed in the direction of the arrows, of the embodiment of thecrown member illustrated in FIG. 5a.

[0025]FIG. 5c is a sectional view substantially similar to FIG. 5b ofanother embodiment of a crown member used in the embodiment of theinvention illustrated in FIG. 2.

[0026]FIG. 6a is a top view of one embodiment of a retaining member usedin the embodiment of the invention illustrated in FIG. 2.

[0027]FIG. 6b is a sectional view, taken along the lines of 6 b-6 b inFIG. 6a and viewed in the direction of the arrows, of the embodiment ofthe retaining member illustrated in FIG. 6a.

[0028]FIG. 7 is an enlarged sectional view of the embodiment of thepresent invention illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, such alterations andfurther modifications in the illustrated device, and such furtherapplications of the principles of the invention as illustrated therein,being contemplated as would normally occur to one skilled in the art towhich the invention relates.

[0030] Referring generally to FIGS. 1 and 2, there is shown oneembodiment of a multi-axial bone anchor assembly 20 of the presentinvention. In the illustrated embodiment, assembly 20 includes areceiver member 30, a bone anchor 50, a crown member 70, and a retainingmember 90. The assembly 20 of the present invention is designed for usewith an elongated member R (FIG. 7) such as a spinal rod, bar or otherorthopedic construct, as further described below.

[0031] Referring now generally to FIGS. 3a-3 d, one embodiment of thereceiver member 30 of the present invention is shown. Receiver member 30defines an upper opening portion 31 a and a lower opening portion 31 b,which in the illustrated embodiment form a single opening 32 extendingthrough receiver member 30 from an upper aperture 33 in top end 34 to alower aperture 35 in bottom end 36. Lower opening portion 31 b ofopening 32, in one specific embodiment, includes a chamber 38 defined bya chamber wall 39. Alternatively, upper and lower opening portions 31 a,31 b can have a variety of configurations, such as each having one ormore sections of differing diameter.

[0032] Opening 32 is partially surrounded by a chamfered or rounded edge40 a at top end 34 of receiver member 30, and is surrounded by chamferedor rounded edge 40 b at the bottom end 36 of receiver member 30.Proximate to bottom end 36, receiver member 30 defines a groove 41 andassociated ledge 41 a around opening 32. In the illustrated embodiment,groove 41 extends around the entire perimeter of opening 32, although itwill be seen that groove 41 could extend only partially around theperimeter of opening 32. Groove 41 has a groove depth A (FIG. 7) and agroove diameter B (FIG. 3a).

[0033] Receiver member 30 in the illustrated embodiment includes a pairof upright branches 42, 43 through which opening 32 extends. Branches42, 43 further define a U-shaped channel 45 transverse to opening 32that communicates with upper portion 31 a and lower portion 31 b ofopening 32, and that accommodates an elongated member R (FIG. 7). In aspecific embodiment, internal threads 44 are formed in branches 42, 43,and branches 42, 43 are provided with indentations or holes 46, whichallow the surgeon to grip receiver member 30 with an appropriate tool(not shown). Internal thread 44 in a specific embodiment is a reverseangle thread, i.e. a thread in which the forward face points down and intoward receiver member 30, as disclosed in commonly-owned U.S. patentapplication Ser. No. 09/188,825, filed Nov. 9, 1998, the disclosure ofwhich is hereby incorporated by reference. Preferably, the top portion47 of receiver member 30 (which includes branches 42, 43) is narrowerthan bottom portion 48 of receiver member 30, thereby reducing the bulkand profile of receiver member 30.

[0034] Referring now generally to FIGS. 4a-4 c, an embodiment of a boneanchor 50 used in the present invention is shown. The illustrated boneanchor 50 is a bone screw, which in one embodiment is substantially likethe bone screw disclosed in U.S. Pat. No. 5,885,286, the disclosure ofwhich patent is hereby incorporated by reference. Bone anchor 50includes an anchorage portion 52 and a head portion 54. Anchorageportion 52 includes at least one thread 56, which may be a cancellousself-tapping thread. Head portion 54 forms part of a sphere in theillustrated embodiment, though alternative curvate and otherconfigurations may be employed. Head 54 in one particular embodimentincludes a series of ridges 58 for improving purchase with the inside ofcrown member 70 (described below). Head 54 may have alternativefriction-increasing surface configuration(s) such as roughening orknurling. Further, head 54 includes a tool-engaging print 60, with whicha tool (not shown) may be engaged to drive anchorage portion 52 into abone. Tool-engaging print 60 is an interior print in the illustratedembodiment, although an exterior print could be used, and it may haveany of a number of configurations, such as hexagonal, hexalobate, orother known torque-transferring configurations.

[0035] Other embodiments of bone anchor 50 are contemplated as beingwithin the scope of the present invention. For example, bone anchor 50could be a bone-engaging hook rather than a screw. In that embodiment,anchorage portion 52 would be configured with a hook rather than anelongated section with thread 56.

[0036] Head 54 of bone anchor 50 is shaped and sized to fit within atleast lower portion 31 b of opening 32 and chamber 38 of receiver member30. Specifically, head 54 has a width that is smaller than the width oflower opening portion 31 b and chamber 38. As more fully describedbelow, bone anchor 50 is inserted into receiver member 30, with head 54entering lower opening portion 31 b and chamber 38 through bottom end 36of receiver member 30.

[0037] Referring now to FIGS. 5a-5 b, there is shown one embodiment ofcrown member 70 of the present invention. In that embodiment, crownmember 70 is in the shape of a circular disc, having an upper surface 72with a beveled edge 74 and a lower surface 78. Lower surface 78 isconfigured to accommodate head 54 of bone anchor 50, and therefore theillustrated embodiment of lower surface 78 has the shape of part of asphere. Alternatively or additionally, the lower surface of crown member70 can have one or more other shapes, such as beveled or conical lowersurface 78′ (FIG. 5c). Lower surface 78 can be provided with a friction-or purchase-enhancing surface configuration (e.g. roughening orknurling) for cooperation with head 54 of bone anchor 50.

[0038] The illustrated embodiment of crown member 70 also includes ahole 80. Hole 80 is provided so that head 54, and specificallytool-engaging print 60, of bone anchor 50 may be accessed through crownmember 70. Crown member 70 is sized and shaped to fit within at leastlower portion 31 b of opening 32 and chamber 38 of receiver member 30.The outer dimension of crown member 70 is preferably slightly smallerthan the inner dimension of chamber 38 and lower portion 31 b of opening32 so that crown member 70 is slidably and rotatably movable withinchamber 38 and opening 32. Further, in the illustrated embodiment theouter dimension of crown member 70 is larger than the inner dimension ofupper opening portion 31 a, so that crown member 70 cannot move intoupper opening portion 31 a.

[0039] Referring now to FIGS. 6a-6 b, there is shown one embodiment ofretaining member 90 of the present invention. In the illustratedembodiment, retaining member 90 has the form of a C-shaped spring orclip defining a gap 91. Retaining member 90 includes a top surface 92and a bottom surface 94. In the illustrated embodiment, retaining member90 also includes internal surfaces 96, 98, 100 that substantiallysurround aperture 102. In one specific embodiment, internal surface 96forms a portion of a sphere of radius substantially identical to theradius of head 54 of bone anchor 50, internal surface 98 is cylindrical,and internal surface 100 is conical and angled outward to allow agreater range of angular positioning of bone anchor 50. In alternativeembodiments, there may be single or multiple internal surfacessurrounding aperture 102, which surface(s) may be cylindrical, conical,spherical or of other appropriate configuration. The diameter ofaperture 102 is smaller than the diameter of head 54 of bone anchor 50and the diameter of crown member 70.

[0040] Retaining member 90 has an unloaded or natural outer diameter D,i.e. a diameter measured when retaining member 90 is under nocontractive (gap-closing) or expansive (gap-opening) stress. Diameter Dof retaining member 90, in one embodiment, is less than groove diameterB of groove 41. Further, retaining member 90 has a body width W that issubstantially constant throughout retaining member 90. Body width W ofretaining member 90 is greater than groove depth A of groove 41.

[0041] Generally referring to FIGS. 1, 2 and 7, assembly 20 is assembledas follows: bone anchor 50, crown member 70 and retaining member 90 areinserted into receiver member 30 through bottom end 36, eitherindividually or substantially in one step. For example, crown member 70may be inserted first, followed by bone anchor 50 with retaining member90 being inserted last. In one specific embodiment, retaining member 90is fitted around bone anchor 50 just below head 54 prior to insertion ofbone anchor 50 into receiver member 30. Retaining member 90 can beplaced around bone anchor 50 by inserting anchorage portion 52 of boneanchor 50 through aperture 102 of retaining member 90 and movingretaining member 90 over anchorage portion 52 toward head 54.Alternatively, gap 91 of retaining member 90 may be pressed against theshank of bone anchor 50 below head 54, so that gap 91 expands to allowplacement of bone anchor 50 within aperture 102 of retaining member 90,whereupon retaining member 90 returns to its original size and shape. Byplacing crown member 70 atop head 54 of bone anchor 50, so that lowersurface 78 of crown member 70 adjoins head 54, and fitting bone anchor50 and retaining member 90 together as described above, simultaneousinsertion of bone anchor 50, crown member 70 and retaining member 90into receiver member 30 can be accomplished.

[0042] Crown member 70 remains slideably and rotatably positioned inlower portion 31 b of opening 32 and/or chamber 38 of receiving member30, and bone anchor 50 remains multi-axially moveable with respect tocrown member 70 and receiving member 30. Retaining member 90 is forcedupward into lower portion 31 b of opening 32. Retaining member 90contracts, making gap 91 smaller, as retaining member 90 is forcedagainst chamfered edge 40 b of receiving member 30, until the outerdiameter of retaining member 90 is the same as the diameter of lowerportion 31 b of opening 32. Retaining member 90 is further advancedalong opening 32 and into groove 41 so that retaining member 90 isfitted into at least a portion of groove 41.

[0043] As noted above, in one specific embodiment the groove diameter Bof groove 41 is smaller than the outer diameter D of retaining member 90in its natural (i.e., unloaded) condition. Thus, when retaining member90 is within groove 41, retaining member 90 presses against the walls ofgroove 41. Alternatively, groove diameter B of groove 41 may be the samesize or slightly larger than the natural outer diameter D of retainingmember 90. In this case, the lower surface 94 of retaining member 90rests upon ledge 41 a of groove 41, and thereby holds retaining member90 within groove 41. Groove depth A of groove 41 is less than the bodywidth W of retaining member 90, so that when retaining member 90 isfitted in groove 41, a portion of retaining member 90 projects intolower opening portion 31 b of opening 32.

[0044] When retaining ring 90 is seated within groove 41, bone anchor 50and crown member 70 are retained within opening 32 of receiver member30. Crown member 70 is supported by head 54 of bone anchor 50, and head54 is supported by internal surface 96 of retaining member 90. Retainingmember 90 is held by groove 41 and/or ledge 41 a of receiver member 30,and thus bone anchor 50 and crown member 70 will not pass throughretaining ring 90 and out of receiver member 30 when retaining ring 90is within groove 41.

[0045] Preferably, assembly 20 is assembled (as described above) priorto use in a surgical procedure. In using the illustrated embodiment ofassembly 20, bone anchor 50 of assembly 20 is threaded into anappropriately prepared hole in a bone (not shown). It will be understoodthat in alternative embodiments of the invention, for example where boneanchor 50 is a bone hook, drilling a hole in bone and threading theanchor therein may not be necessary. Threaded anchoring portion 52 isinserted into the hole, and an appropriate screwing tool is used withtool-engaging print 60 of bone anchor 50 through hole 80 in crown member70, and bone anchor 50 is threaded into the bone. When bone anchor 50has been threaded into the bone to the desired depth, receiver member 30is positioned so that opening 32 forms a desired angle with bone anchor50, as depicted in FIG. 1. In the illustrated embodiment, the angle θbetween bone anchor 50 and opening 32 can be any value up to 30 degreesin any direction. It will be seen that the maximum angle of bone anchor50 relative to opening 32 can be changed in several ways, for example bythinning the portion of bone anchor 50 beneath head 54, by providingsteeper angulation of chamfered edge 40 b, and/or by placing groove 41as close as possible to bottom end of 36 of receiver member 30.

[0046] As described above, receiver member 30 may be angled as thesurgeon desires with respect to bone anchor 50. An elongated member Rsuch as a spinal rod, connector, or other orthopedic surgical implant iscoupled with assembly 20. Elongated member R is placed in channel 45 ofreceiver member 30, and contacts top surface 72 of crown member 70. Acompression member 120, such as a set screw or threaded plug, isthreaded into threads 44 of receiver member 30 and down onto elongatedmember R. Compression member 120, in one embodiment, is a set screw orplug having external threads 122 and a print 124 for applying torque,and in a specific embodiment is a break-off set screw as disclosed inU.S. Pat. No. 5,885,286 to Sherman et al., incorporated herein byreference. In a further embodiment, thread 122 is a reverse angle threadas disclosed in U.S. patent application Ser. No. 09/188,825, filed Nov.9, 1998, incorporated herein by reference, which is compatible with thereverse angle embodiment of thread 44 of receiver member 30, describedabove. Alternatively, where receiver member 30 is externally threaded,compression member 120 could be an internally-threaded nut.

[0047] As compression member 120 is tightened, elongated member R isforced downward against crown member 70, which pushes crown member 70down onto head 54 of bone anchor 50. Head 54 is thereby clamped betweenretaining member 90 and crown member 70. In the embodiment of theinvention in which head 54 includes ridges 58, ridges 58 are pressedinto lower surface 78 of crown member 70. In this way, bone anchor 50 islocked into the desired angular position with respect to elongatedmember R and the remainder of assembly 20.

[0048] Alternatively, assembly 20 can be assembled during the surgicalprocedure. Bone anchor 50, with retaining ring 90 already positionedbeneath head 54, is inserted into the bone. Crown member 70 is placedatop bone anchor 50 or in opening 32 in receiver member 30. Receivermember 30 is then pressed down onto head 54 of bone anchor 50, forcingretaining ring 90 to contract, to enter opening 32, and to seat ingroove 41 as described above. After assembly 20 is assembled in thisfashion, an elongated member is loaded into receiver member 30 andlocked as previously described.

[0049] Preferred materials for the present invention include stainlesssteel and titanium. It will be recognized that any sturdy biocompatiblematerial may be used to accomplish the osteosynthesis and otherorthopedic surgical goals of the present invention. In one specificembodiment, crown member 70 may be made of a material somewhat softerthan the material used for ridges 58 of head 54 of bone anchor 50. Suchconstruction will allow ridges 58 to penetrate somewhat more easily intointerior surface 78 of crown member 70 during locking of assembly 20,thereby providing a more definite purchase between ridges 58 and crownmember 70. In another specific embodiment, crown member 70 may be madeof a material somewhat softer than the material used for elongatedmember R. Such construction will allow upper surface 72 of crown member70 to deform to the shape of elongated member R during locking ofassembly 20, also providing a more secure locking of the implant.

[0050] While the invention has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only the preferred embodiment has been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. A bone anchor assembly for engagement to anelongated member, comprising: a receiver member defining an upperopening portion and a lower opening portion each having respectiveminimum widths, a channel configured to receive the elongated member andcommunicating with said upper opening portion and said lower openingportion, and a groove around a portion of said lower opening portion; acrown member movably disposed in said lower opening portion, said crownmember including an upper surface and a lower surface; a bone-engaginganchor having a lower portion configured to engage a bone and a headhaving a width, said width of said head being smaller than said minimumwidth of said lower opening portion, said head being movably disposed insaid lower opening portion adjacent to said lower surface of said crownmember; and a retaining member defining an aperture smaller than saidwidth of said head, said retaining member at least partially housed insaid groove of said receiver member and positioned around said anchorand below said head, wherein said retaining member prevents removal ofsaid head from said lower opening portion.
 2. The assembly of claim 1,wherein said upper opening portion and said lower opening portion format least part of a single opening through said receiver member.
 3. Theassembly of claim 2, wherein said receiver member includes two brancheswhich define said upper opening portion and said channel.
 4. Theassembly of claim 3, wherein said branches include internal threads. 5.The assembly of claim 4 further including a compression memberthreadedly connected to said internal threads.
 6. The assembly of claim2, wherein said receiver member defines a chamber that forms at least apart of said lower opening portion, and said crown member being movablydisposed within said chamber.
 7. The assembly of claim 6, wherein saidanchor is a bone screw.
 8. The assembly of claim 7, wherein said head ofsaid bone screw is at least partially spherical.
 9. The assembly ofclaim 8, wherein said head of said bone screw includes ridges.
 10. Theassembly of claim 8, wherein said lower surface of said crown member isat least partially spherical.
 11. The assembly of claim 10, wherein saidlower surface of said crown member includes a roughened portion.
 12. Theassembly of claim 6, wherein said crown member has a width greater thansaid upper opening portion of said receiver member.
 13. The assembly ofclaim 12, wherein said head of said bone anchor includes a tool-engagingprint.
 14. The assembly of claim 13, wherein said crown member defines ahole through said upper surface through which said head of said boneanchor can be accessed.
 15. The assembly of claim 12, wherein saidretaining member is a C-shaped member.
 16. The assembly of claim 15,wherein said retaining member has an unloaded outer diameter, saidreceiver member has a groove diameter, and said unloaded outer diameterof said retaining member is greater than said groove diameter of saidreceiver member.
 17. The assembly of claim 16, wherein said retainingmember has a body width, said groove has a groove depth, and said bodywidth is greater than said groove depth.
 18. The assembly of claim 17,wherein said retaining member includes an inner concave surface forengaging said head of said bone anchor.
 19. The assembly of claim 18,wherein said inner concave surface forms part of a sphere.
 20. Theassembly of claim 2, wherein said lower surface of said crown member isbeveled.
 21. The assembly of claim 2, wherein said lower surface of saidcrown member is concave.
 22. The assembly of claim 2, wherein said crownmember has a width greater than said upper opening portion of saidreceiver member.
 23. The assembly of claim 22, wherein said crown memberdefines a hole through said upper surface through which said head ofsaid bone anchor can be accessed.
 24. The assembly of claim 2, whereinsaid retaining member is a C-shaped member.
 25. The assembly of claim24, wherein said retaining member has an unloaded outer diameter, saidreceiver member has a groove diameter, and said unloaded outer diameterof said retaining member is greater than said groove diameter of saidreceiver member.
 26. The assembly of claim 24, wherein said retainingmember has a body width, said groove has a groove depth, and said bodywidth is greater than said groove depth.
 27. The assembly of claim 26,wherein said retaining member includes an inner concave surface forengaging said head of said bone anchor.
 28. The assembly of claim 27,wherein said inner concave surface forms part of a sphere.
 29. A bonefixation apparatus comprising: an elongated member configured forplacement adjacent and along a length of at least one bone; a receivermember defining an opening therethrough from a top end to a bottom end,said opening having a lower aperture at said bottom end and an upperaperture at said top end, said receiver member also defining a groovearound a portion of said opening, said groove being proximate to saidlower aperture, said receiver member further including a channelcommunicating with said opening and said upper aperture, said channelbeing configured to receive said elongated member therein; a crownmember insertable through said lower aperture and disposed within saidopening, said crown member having a lower surface and an opposite uppersurface contacting said elongated member; a bone anchor having a lowerportion configured for engaging a bone and a head having a widthdimension, said head being insertable through said lower aperture andadjacent to said crown member within said opening of said receivermember; a retaining member defining an aperture having a width dimensionthat is less than said width dimension of said head, said retainingmember being positioned around a portion of said bone anchor and atleast a portion of said retaining member being housed within at least aportion of said groove; and a compression member engaged within saidopening proximate to said upper aperture, said compression memberoperable to press said elongated member against said crown member,thereby fixing said head of said fastener between said crown member andsaid retaining member.
 30. The apparatus of claim 29, wherein saidelongated member is a spinal rod.
 31. The apparatus of claim 29, whereinsaid retaining member is a C-shaped member.
 32. The apparatus of claim31, wherein said retaining member has an unloaded outer diameter, saidreceiver member has a groove diameter, and said unloaded outer diameterof said retaining member is greater than said groove diameter of saidreceiver member.
 33. The apparatus of claim 32, wherein said retainingmember has a body width, said groove has a groove depth, and said bodywidth is greater than said groove depth.
 34. The apparatus of claim 33,wherein said retaining member includes an inner concave surface forengaging said head of said bone anchor.
 35. The apparatus of claim 34,wherein said inner concave surface forms part of a sphere.
 36. Theapparatus of claim 29, wherein said crown member defines a hole throughsaid upper surface through which said head of said bone anchor can beaccessed.
 37. The apparatus of claim 29, wherein said head of said boneanchor is at least partially spherical.
 38. The apparatus of claim 37,wherein said lower surface of said crown member is at least partiallyspherical.
 39. An apparatus for receiving and holding components of amulti-axial bone anchor system, comprising a member defining an upperopening portion and a lower opening portion, a channel transverse to andcommunicating with said upper opening portion and said lower openingportion, and a groove around at least a portion of said lower openingportion.
 40. The apparatus of claim 39, wherein said upper openingportion and said lower opening portion form at least part of an openingthrough said member from a top end to a bottom end.
 41. The apparatus ofclaim 39, wherein said groove is proximate said bottom end of saidmember.
 42. The apparatus of claim 40, wherein at least a portion ofsaid upper opening portion is threaded.
 43. The apparatus of claim 41,wherein said member includes two branches that define said upper openingportion and at least a portion of said channel.